This invention relates to apparatus for preventing arc discharge of a transfer switch circut, especially used for switching an inductive load.
When opening an inductive load, for example, an electric motor, by a relay or contactor, an excessive arc discharge occurs across the contacts of the relay. In a prior art transfer switch circuit, as shown in FIG. 1, a source 3 is connected between opposing stationary contacts 1 and 2, and an electric motor 5 is conected to a movable contact 4 movable between the stationary contacts 1 and 2. When the movable contact 4 is transferred from the stationary contact 1 to the stationary contact 2 by a snap action, the eletromagnetic energy stored in the inductance L of the motor 5 creates electric discharge between contacts 1 and 4. Such discharge persists by the energy fed from the source 3 even after contact 4 engages stationary contact 2.
For this reason, it has been the practice to connect across contacts an arc preventing element, such as a resistor or a capacitor having a suitable rating for preventing melting or welding of the contacts.
However, in an environment where the temperature condition is severe as in a transfer switch circuit carried by a vehicle, the reliability of a conventional oil filled capacitor is low, and a mylar capacitor is difficult to construct to have a large capacity although its capacitance variation is relatively insensitive to temperature variation, so that these capacitors are not suitable for being used as an arc preventing element of a transfer switch circuit utilized for a large inductive load.